In some patients, the arthritis that affects the inside of a knee may not affect all of the surfaces of the bones. Surgeons tend to consider knees as having three compartments; the medial compartment (between the femur and the tibia on the inside/medial side of the knee), the lateral compartment (between the femur and the tibia on the outside/lateral side of the knee) and the patellofemoral compartment (between the back of the patella/kneecap and the front of the knee – the trochlea).

If arthritis within a knee affects only one compartment, then there are some instances where a partial knee replacement might be appropriate.

Partial replacements of the medial compartment of the knee are referred to as medial unicompartmental knee replacements. Partial replacements of the lateral compartment of the knee are referred to as lateral unicompartmental knee replacements (although these are rarely done). Replacements of the patellofemoral compartment are called patellofemoral replacements.

X-ray of a medial uni-compartmental knee replacement.

The results of partial knee replacements are not quite as good as they are for a total knee replacement, because there is always the risk of a patient developing arthritis in the rest of the knee, and thus requiring further surgery with a total knee replacement (which would count as a ‘failure’ when looking at the results of the original surgery). However, partial knee replacements generally involve smaller, less invasive surgery than a total knee replacement, and so recovery is generally quicker and early complications generally lower. Furthermore, conversion of a partial knee replacement to a total knee replacement at a later date is not too technically demanding, and thus partial knee replacements are sometimes seen as a good alternative for younger patients whose arthritis is localised to one particular area of the knee.

TKR is a major procedure, which is normally undertaken under a general or a spinal anaesthetic. The operation lasts in the region of 1 to 1 hours, and normally requires a hospital stay of 4 to 10 days, depending on the patient’s age and general fitness. It can then take anything up to 3 months for the muscles around the knee to begin to heal up fully and for patients to begin to feel the real benefit of the surgery.

The main potential complications of the surgery are:

Infection

Bacteria are carried in the air, and are found in our skin, in our mouths, up our noses and all around us. There is a small risk of bacteria getting into a knee at the time of a knee replacement, and causing an infection.

Superficial infections, affecting just the wound and the skin, can often be treated well with just a course of antibiotics. However, deeper infections can be very serious, as the bacteria can get into the bone (causing osteomyelitis) or they can stick onto the surface of the metal of the knee replacement. If the bacteria do stick to the metal of a TKR, then they secrete a sticky membrane around themselves that makes it very difficult for antibiotics to get to them. For this reason, deep infections in a joint replacement can be a very serious complication. In order to treat such deep infections, it may be necessary to remove the knee replacement completely. If the infection then clears up, then it might be possible to put in a new, revision knee replacement. Otherwise, it is sometimes necessary to fuse the knee (an arthrodesis).

We take the risk of infection very seriously, and all TKRs are performed in a laminar airflow theatre, with ultra-filtered air, and full aseptic precautions. Also, patients are given prophylactic antibiotics immediately before, and for two doses after the surgery, to help minimise the risk.

DVTs / PEs

After any major surgery or trauma, the blood becomes more likely to clot. In addition, after surgery on the knee, the leg is less mobile and the blood in the veins may flow more slowly. There is, therefore, a significant risk of patients developing blood clots in the veins of the leg after a knee replacement. A deep vein thrombosis (DVT) is a blood clot within the veins deep in the leg (as opposed to the superficial veins just under the skin). Small DVTs are common and often cause little or no problems. However, larger DVTs can cause painful swelling of the leg, with potential long term complications such as persistent swelling or ulceration of the leg. Clots may also break off larger DVTs and can go off into the circulation and end up lodged in the lung – a pulmonary embolus (PE) – causing chest pain and shortness of breath. Large PEs can, on rare occasions, even be fatal.

The risk of DVTs is therefore taken very serious after TKR. First, patients are encouraged to move their leg and walk as early as possible. When the calf muscles contract they pump the blood through the veins, increasing the flow and therefore decreasing the risk of thrombosis. Patients are therefore told to keep their feet moving by lifting the toes up and pointing the toes down repeatedly after the surgery. In addition, most patients are given low molecular weight heparin (LMWH) injections via a small needle into the skin once a day whilst in hospital after their surgery. The LMWH thins the blood slightly and again reduces the risk of DVT.

Despite the best precautions, the risk of DVT cannot be totally eliminated. If a DVT is suspected post-operatively, then patients are sent for an ultrasound scan of the leg. If a DVT is seen, then a patient may be started on warfarin tablets to thin the blood, and depending on the size of the clot and the patient’s individual risk factors, they may be kept on warfarin tablets for several months, which requires regular blood tests to check the warfarin levels.

In cases where a patient’s knee is out of line, the weight will not be distributed evenly within the knee, and this may cause excessive wear and pain on one side of the knee. In such cases, it may be possible to realign the knee by cutting the bone (most frequently the top of the tibia), realigning it and fixing it with a metal plate and screws. This is called a realignment osteotomy, and it can give good pain relief and significantly delay the time when a total knee replacement might be needed.

In rare cases, where the damage to a knee joint makes it too difficult to contemplate putting in a knee replacement, there are occasional instances where fusion of the knee joint (arthrodesis) by fixing the tibia and the femur together with the knee slightly bent, can be a good option. Although the knee will then not bend, the operation does remove the pain in a severely arthritic knee and means that a patient can fully weight bear and thus walk, albeit with a limp. Whereas arthrodesis used to be a more common operation in the past, it is only rarely required nowadays.